Sunday, August 10, 2014

Northern Convergence: The Olympics, Where a Trench Became Sky-Piercing Peaks

Our journey through Western Canada and the Pacific Northwest began as we met with our students in the Seattle area one evening in late July. After a complicated couple of hours of meetings and negotiating van rentals (reservations three months earlier are only the preliminaries), we settled in for the night, and prepared to hit the road at 6:30 AM. We had a long day ahead that wouldn't end until we rode the last ferry across the Strait of Juan de Fuca, arriving in Victoria at 11 PM. It wasn't the kind of itinerary I like, but in the end it worked out (as noted in one of the few posts I was able to complete during the trip itself).

We wove through the morning traffic of Tacoma and Olympia (luckily most everyone was headed the opposite direction). We reached the village of Blyn near Port Angeles by 9 AM or so where we stopped at a roadside rest to have our first introductory presentations on the geology and anthropology of the Puget Sound and Olympic Peninsula. The skies were somewhat overcast, which was worrisome because a storm was rolling in soon, and we had hopes of having a clear view of the Olympic Mountains from Hurricane Ridge.
Mrs. Geotripper and I had paid a visit to the ridge a few days earlier and were treated to spectacular views, and I would have been heartbroken if we had showed up with the students to a fogged-in viewpoint. It's happened before; you can ask my students of their memories of seeing Mt. St. Helens in 2011 and get blank stares. We spent most of a day at St. Helens looking at fog, rain, and a few downed trees next to the highway. I didn't want such a thing to happen again, so I was tense as we started the climb out of Port Angeles towards Hurricane Ridge. In fog.

To my great relief, we broke through the clouds and drove into bright sunlight. We climbed the winding highway through the thick forest with increasingly far-ranging views. But nothing quite prepares anyone for the view from the end of the paved road at Hurricane Ridge. It is simply astounding.
The Olympic Mountains are geologically distinctive, to say the least. The mountains rise from sea level to nearly 8,000 feet and are extremely rugged. They capture prodigious amounts of rain and snow on the western flanks, so much so that temperate rainforests coat the western slopes. It would have been a nightmare for geologists who were trying to unravel the geologic history.

Subduction is the story of the Pacific Northwest. For most of 200 million years a convergent boundary has been active in the region, as the crust of the Pacific Ocean basin has been sinking against the edge of the North American Continent. In some places, for instance California, the subduction zone has been replaced by a transform boundary (the San Andreas fault). But in Northern California, Oregon, Washington, and part of British Columbia, the subduction zone is still active, still producing earthquakes, and still raising mountains. It's called the Cascadia Subduction Zone
Source: Geological Society of America

In a "normal" subduction zone, there are four parts: the trench, an accretionary wedge, a forearc basin, and a magmatic arc. The trench is the deepest part of the ocean floor where the oceanic crust sinks back into the mantle. The accretionary wedge is a collection of seafloor sediments and crust that has been scraped off the subducting plate and added to the edge of the continent. The forearc basin is a shallow sea that may develop inland of the accretionary wedge (California's Great Valley originated in this fashion). The magmatic arc is a system of volcanoes and intrusive plutons resulting from the melting of rocks in the lower crust and upper mantle above the descending slab (water released from the slab lowers the melting point of the rock, leading to the formation of the molten rock). Other features may develop, depending on the angle of subduction or the geometry of the plate boundary. There will be more on those later in the series.

Looking at the thickly forested slopes below Hurricane Ridge, I cannot envy the geologists who originally mapped the Olympic Mountains. Simply finding an exposure of rock must have been challenging at times. What these geologists did was to take the rare rock exposures and extrapolate them into a semi-coherent map that reveals the structure of the Olympic Mountains. They did the equivalent of taking a few pieces of a jigsaw puzzle, putting them in the right location relative to the others, and then drawing in the remainder of the puzzle from scratch. I've been way too spoiled by the naked rock exposures of places like Death Valley and the Mojave Desert!

The geologic map reveals the basic structure of the Olympics. A "horseshoe" of basalt and sedimentary rocks (the Peripheral Rocks, or Crescent Formation) partially surrounds the "Core Rocks", an assemblage of lightly metamorphosed sandstone and shale layers. The Core Rocks are characteristic of the types of deposits that form from underwater landslides ("turbidity currents") within the trench and accretionary wedge of a subduction zone. The fact that these rocks are now thousands of feet above sea level is the interesting conundrum. Accretionary wedges are generally below sea level, or exist as small islands. They can be pushed higher. For instance, the rocks of the Cascadia accretionary wedge are exposed in the Coast Ranges of  Washington, Oregon and California, but nowhere are the exposures as spectacular as the Olympic Mountains.
Convergent boundaries can be exceedingly complex places. Bits and pieces of continents and island arcs may randomly arrive at the subduction zone, mucking up the subduction process the way too many sheets of paper at once can muck up a paper-shredder. In the case of the Olympics, there was a mass of land north (Vancouver Island) and an accreted terrane to the south (the North Cascades), and a bend in the subduction zone itself. In essence, too much material was being stuffed into the subduction zone, so the excess material went the only way it could, which was up (see my earlier post on this subject, "Sorry, this trench is full..."). The mountains have been rising for around 15 million years. They would be higher, but the incredible amount of precipitation tears the mountains down at a roughly equivalent rate.
The basalt is exposed along the Hurricane Ridge Road and along trails near the viewpoint. The slopes below Hurricane Ridge also include exposures the intensely folded shale layers.

When we first visited Hurricane Ridge the prior week, the highest peaks were obscured by clouds. When the class arrived, the mountains were clear and we could easily observe the glaciers that scour the upper reaches of the mountains. Glaciers technically shouldn't exist here. Although we were at a high enough latitude, the nearby Pacific Ocean moderates the climate, keeping things warmer than they would otherwise be (the Olympics are at the same latitude as Great Falls, Montana, or St. Paul, Minnesota). But temperature isn't the only factor in glacier development. The sheer amount of snowfall in combination with temperatures that are just cold enough allows glaciers to exist at these unusually low elevations.
We had a good introduction to the basic features of alpine glaciation as we gazed across the valley to Mt. Olympus. We could clearly see bergshrunds (the cracks that develop at the top of glaciers where they pull away from cliffs), and crevasse fields in the lower parts where the glaciers flowed over obstructions. There were horns, aretes, and cirques as well. Glaciers were going to be a big part of the story of British Columbia, and Hurricane Ridge provided a spectacular setting for the first discussion of how they worked.
We gave the students some time to wander the network of trails around the visitor center, and I set off to Sunrise Point to get a panoramic view. I was so relieved that the storm had not yet arrived, but of course it was still out there, and there would a few consequences for our trip. But not on this most beautiful of days.

We headed down to Port Angeles for lunch and to find to road to the other major locale the day: Neah Bay and lands of the Makah people.

1 comment:

Anonymous said...

Interesting site, and i have bookmarked it...I want to read through all the info at my leisure! :)

This page caught my attention, I have a small chunk of property right next to the Olympic Mountains above Lake Cushman. In your geologic map, it appears to be right inbetween the purple/basaltic & yellow/sand/gravel formations.

In my spare time I am trying to learn enough to identify what appears to be an 'erratic' boulder on my property, that washed out of the creek after a flood several years ago.

It is a polished greenish rock (the size of a large backpack, *very* dense and heavy, almost impossible to move w/out block and tackle) but it is such a pretty green color.

Some day i hope I can identify it. My mom thinks it is 'green schist'.

Thanks for the work you have done here! :)